The present invention relates to the field of digital telephony. In particular, the present invention relates to accessing the data stream between a digital telephone and the switch.
In the world of digital telephones there are many different service providers. Each provider has a proprietary method for encoding the communication stream that runs between the telephone and the switch or PBX (private branch exchange). Most communications between the digital phone and the switch work by sending signals having different waveforms of data between the phone and the switch, typically on a two-wire pair, with the data digitally encoded in the signal.
It is common in digital phone systems for the information to be transmitted between the switch and phone using a framing scheme or protocol. Typically, frames are transmitted on 125 microsecond intervals. The format of the frames varies from protocol to protocol but the format of every frame in a protocol is the same. For example one telephony protocol may dictate that the format of its data frames contain a start bit, eight bits of voice data, two bits of control data, two bits of secondary voice data, and a parity bit. Each data frame transmitted using that particular protocol will have that format. Another protocol may require a format for its data frames containing, for example, a start bit, sixteen bits of voice data, two signaling bits, eight data bits, an auxiliary bit, and a stop bit. Each data frame in a particular proprietary data stream will have the format required by the protocol for that data stream. Depending on the protocol there will typically be from 20 to 100 bits of information in each telephony data frame.
There are certain products available that are connected on the telephone line between the telephone and the switch. For example, a digital telephone may be connected to a personal computer (PC) that is connected to the telephone line. The PC is situated on the telephone line between the telephone and the switch. The PC can be used to place phone calls, answer the phone, voice mail, or fax. In order to do these things, the PC must emulate the communication protocol between the telephone and the switch.
Devices, such as the PC example above, that passively tap the switch/phone line must reliably create a signal that demarcates the start of each frame. One of the key elements in implementing a telephony protocol stack is frame synchronization, i.e., locking the hardware to the frame frequency and phasing. Once this has been accomplished, the task of extracting and regenerating the telephony information becomes possible.
In the prior art, frame synchronization relies on protocol specific hardware implementations. The implementation of a new protocol requires a new hardware design and the integration of the hardware into existing systems. It often takes months to design and implement the new hardware; thus the need to change the hardware design for each telephony protocol is an expensive and time-consuming problem.